The use of alkali metal borohydrides such as sodium borohydride and potassium borohydride as blowing agents in connection with the production of cellular articles is well known. For example, U.S. Pat. No. 2,951,819 utilizes an alkali metal borohydride in connection with the manufacture of cellular articles. U.S. Pat. No. 3,167,520 relates to cellular articles prepared from polystyrene by extruding polystyrene resin containing an alkali metal borohydride which decomposes during extrusion to form a cellular article. The invention of this application is directed to an improvement to the technique of aforesaid U.S. Pat. No. 3,167,520.